This invention relates to the field of electrostatic imaging and, more particularly, to the preparation of liquid toners containing components for imparting chargeability to ordinarily unchargeable liquid toner particles, enhancing the chargeability of insufficiently chargeable liquid toner particles, and controlling the polarity of liquid toner particle charge.
In the art of electrostatic photocopying or photoprinting, a latent electrostatic image is generally produced by first providing a photoconductive imaging surface with a uniform electrostatic charge, e.g. by exposing the imaging surface to a charge corona and then selectively discharging the surface by exposing it to a modulated beam of light corresponding, e.g., to an optical image of final image to be produced. This forms a latent electrostatic image having a xe2x80x9cbackgroundxe2x80x9d portion at one potential and a xe2x80x9cprintxe2x80x9d portion at another potential. The latent electrostatic image can then be developed by applying to it charged pigmented toner particles, which adhere to the print portions of the photoconductive surface to form a toner image which is subsequently transferred by various techniques to a final substrate (e.g. paper).
It will be understood that other methods may be employed to form an electrostatic image, such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. The charge may be formed from an array of styluses. It is to be understood that the invention is applicable, generally to both printing and copying systems.
In liquid-developed electrostatic imaging, the toner particles are usually dispersed in an insulating non-polar liquid carrier such as an aliphatic hydrocarbon fraction, which generally has a high-volume resistivity above 109 ohm cm, a dielectric constant below 3.0 and a low vapor pressure (less then 10 torr. at 25xc2x0 C.). The liquid developer system further comprises so-called charge directors, i.e. compounds capable of imparting to the toner particles an electrical charge of the desired polarity and uniform magnitude.
In the course of the process, liquid developer is applied to the photoconductive imaging surface. Under the influence of the electrical potential present in the latent image and a developing electrode which is usually present, the charged toner particles in the liquid developer migrate to the print portions of the latent electrostatic image, thereby forming the developed toner image.
Charge director molecules play an important role in the above-described developing process in view of their function of controlling the polarity and magnitude of the charge on the toner particles. The choice of a particular charge director for use in a specific liquid developer system, will depend on a comparatively large number of physical characteristics of the charge director compound, inter alia its solubility in the carrier liquid, its chargeability, its high electric field tolerance, its release properties, its time stability, the particle mobility, etc., as well as on characteristics of the toner and the development apparatus. All these characteristics are crucial to achieve high quality imaging, particularly when a large number of impressions are to be produced.
A wide range of charge director compounds for use in liquid-developed electrostatic imaging are known from the prior art. Examples of charge director compounds are ionic compounds, particularly metal salts of fatty acids, metal salts of sulfo-succinates, metal salts of oxyphosphates, metal salts of alkyl-benzenesulfonic acid, metal salts of aromatic carboxylic acids or sulfonic acids, as well as zwitterionic and non-ionic compounds, such as polyoxyethylated alkylamines, lecithin, polyvinyl-pyrrolidone, organic acid esters of polyvalent alcohols, etc.
Desired physical characteristics of toner particles is that they have softening points consistent with the temperature capabilities of the final substrate, good adhesion to the substrate and abrasive resistance. To this end toner particles are often formed of polymer materials having these properties and having pigments dispersed therein or which are otherwise colored.
Unfortunately, many polymers which would make ideal toner materials are difficult if not impossible to charge to a level which is useful in an electrostatic imaging process.
U.S. Pat. No. 4,526,852 (Herrmann et al) used a particulate acid or ester wax derived from montan wax, hydrated castor oil or polyoctadecene to reduce the specific electrical conductivity of a liquid developer containing negatively charged toner particles.
Notwithstanding the undoubted utility of charge directors, and the various attempts which have been made to improve their performance, from one aspect their use depends on the toner particles having a surface which is receptive to the application of charge directors. In other words, the art would have considered certain types of particles either virtually unchargeable or insufficiently chargeable. Moreover, it may be desirable to change the polarity of the charged particles from that which is conventionally associated with a particular charge director, when used in conjunction with a particular type of toner particle.
It is an object of the present invention to provide improved liquid toner compositions containing charge directors, which address the problems mentioned in the preceding paragraph. Other objects of the invention will appear from the description which follows.
The present invention accordingly provides in one aspect, a liquid toner for electrostatic imaging which comprises:
an insulating non-polar carrier liquid;
at least one charge director; and
toner particles dispersed in the carrier liquid, the particles comprising:
a core material which is unchargeable or weakly chargeable by the at least one charge director, but which is are otherwise suitable for use as a toner material; and
a coating of at least one ionomer component in an amount effective to impart enhanced chargeability to the ordinarily unchargeable or weakly chargeable particles.
In a second aspect of the invention, there is provided liquid toner for electrostatic imaging which comprises:
an insulating non-polar carrier liquid;
at least one charge director; and
toner particles dispersed in the carrier liquid, the toner particles comprising:
a core material which is chargeable to a first polarity by the at least one charge director; and
a coating of at least one ionomer component in an amount effective, together with the at least one charge director, to impart a charge having a polarity different from the first polarity to the coated particles.
In a further aspect of the invention, a method of producing liquid toner for electrostatic imaging, which method comprises dispersing particles in an insulating non-polar carrier liquid, and mixing also at least one ionomer with the liquid. Preferably, the mixture is first heated to a temperature at which the ionomer dissolves in the carrier liquid and then cooled to a temperature whereat the ionomer is not soluble in the carrier liquid, thereby coating the particles with ionomer material.
In a preferred embodiment of the invention, the mixture is agitated at least during the step of cooling.
Preferably, at least one charge director is added to the mixture, most preferably after the step of cooling.
According to a preferred embodiment of the invention, the particles are formed of a material which, in the presence of charge director alone, is ordinarily unchargeable or weakly chargeable, but is otherwise suitable for use as toner particles, and the at least one ionomer component is used in an amount effective to impart enhanced chargeability to the toner particles.
In a preferred embodiment of the invention, the at least one ionomer component is used in an amount effective to reverse the polarity conventionally imparted to the material of the particles by the at least one charge director.
In still another aspect, the present invention provides an electrostatic imaging process which comprises the steps of: forming a charged latent electrostatic image on a photoconductive surface; applying to said surface charged colorant particles from a liquid toner of the invention (or as prepared by the method of the invention); and transferring the resulting toner image to a substrate.
Generally, the ionomers utilized as coatings in the Examples herein are low molecular weight ionomers which are generally considered to be too soft to be used alone for toner materials.